Glazed flat plate solar collectors offer a simple and cost effective approach to heating water. However, on clear nights solar collectors can transfer a significant amount of heat to the atmosphere by radiation. In areas with a cool climate and high relative humidity the radiation heat loss can lead to the temperature of the glazing often reaching the dew-point of the surrounding air, this leads to condensation developing on the glazing. If water condenses on the inside of the glazing and repeatedly drips onto the solar absorber this could lead to damage of the solar collector surface thus shortening its operating life. Also, the build-up of condensation on the glazing can lead to the growth of mould, is visually displeasing to the owner, and means energy must be “wasted” to evaporate the moisture during the following day. This study aims to develop the understanding of the role that condensation plays on collector performance, as well as addressing ways of minimising the impact it has. In doing so, it uses numerical modelling and experimental testing to determine the frequency of condensation in glazed flat plate solar water heaters under typical operating conditions. It shows that climatic factors including relative humidity, ambient temperature and wind speed determine the frequency of condensation for any given location. However, it also reveals that the frequency of condensation can be modified by altering the convection heat transfer coefficient inside the collector and by using low emissivity coatings on the glazing layer.